1. Technical Field
The invention relates to methods of manufacturing uranium oxide base pellets for use as nuclear fuel and more particularly to manufacture of "green" pellets by cold compression of a powder whose major component is uranium dioxide.
The invention makes use of uranium dioxide powder obtained by a dry process (i.e., by direct reduction of UF.sub.6 into dioxide). The powders obtained by the dry conversion process, described in French No. 2,060,242 and U.S. Pat. No. 4,397,824, including water vapor hydrolysis followed by pyrohydrolysis of the uranyl fluoride UO.sub.2 F.sub.2 obtained, have the advantage of being readily sinterable. On the other hand, the green pellets obtained by compression are relatively fragile. Handling thereof is delicate; the rejects are numerous if special care is not exercised.
2. Prior Art
Different methods have been proposed for reducing the fragility of green pellets. Attempts have been made to increase the density in green condition by increasing the compacting pressure, which has the drawback of causing premature wear of the compression dies. It has been proposed to add a binder and to granulate the powder and/or to subject the powder to different treatments (French 2,561,026). All of these solutions have drawbacks. Those which use water or a binder raise criticality problems. The methods are generally applicable only to small batches, practically not exceeding 50 kg.
The fragility of the green pellets obtained by compression of UO.sub.2 powder obtained by dry conversion may probably be attributed to the fact that the powder is formed of fine crystallites more or less rounded, with a very low oxygen over-stoechiometry (ratio O/U of from 2.02 to 2.06). Reoxidation of the powder improves the strength of the pellets. Surface oxidization of the uranium oxide UO.sub.2 redivides the grains into jagged aggregates with intertwining ramifications. The powder then has properties comparable to those of a powder obtained by a wet process: during compression of the pellets into shape, the grains engage each other and provide coherence.
But limited surface oxidization to UO.sub.2 powder is difficult to achieve industrially. Oxidation tends to bolt until the whole of the oxide has oxidized to U.sub.3 O.sub.8 which is difficult to sinter into pellets free of cracks, unless a slow controlled reduction step is added to the manufacture during presintering.
Moreover, oxidation generates islets of U.sub.3 O.sub.8 which, during sintering, shrink more than the UO.sub.2 grains, whence a heterogeneous texture. It is moreover practically impossible to carry out oxidation in a reactor for direct conversion of UF.sub.6, such as described in U.S. Pat. No. 4,397,824.
It is, moreover, current practice to recycle waste from manufacture of the pellets, such as the oxidized grinding muds, the rejected pellets, the splinters and residues of powders in the form of U.sub.3 O.sub.8 (French 2,001,113). These oxidized powders, while they slightly improve the strength of the green pellets if mixed with UO.sub.2 before compacting and sintering, have a very unfavorable effect on the density and texture of the sintered pellets, for the U.sub.3 O.sub.8 is in the form of dense and coarse grains. The percentage of recycled U.sub.3 O.sub.8 in the green pellets is generally limited to 12% wt. at most.
It has further been suggested to mix UO.sub.2 obtained by a wet process with an amount of U.sub.3 O.sub.8 prepared for that purpose and whose function is to generate pores which collect fission gases in the sintered pellets (French No. 1,412,878).